Communication with E-mails through the Internet has rapidly been spread worldwide. E-mails including common text data and audio data can be transmitted and received by personal computers including a modem and a Local area network (LAN) interface coupled to a network such as the Internet or the LAN. Other communication apparatuses with display such as facsimiles coupled to a telephone network can transmit and receive E-mails through providers.
Japanese Patent Laid-open Publication 11-205457 discloses a communication system where E-mails are displayed on a character display of each extension telephone linked via a computer telephony integration (CTI) interface to a private branch exchange (PBX) without a mail server. A conventional PBX has a function of transferring an E-mail received from the outside to one of extension telephones in addition to a basic function of speaking with other the extension telephones. However, such a conventional PBX may be disadvantageous for users to easily operate it to exchange E-mails between the extension telephone and an outline telephone.